The coherent combining of laser sources is particularly applicable to the production of high-power laser sources and/or, in the case of ultra-short pulsed sources, high-energy laser sources, for example with a pulse width of less than a picosecond.
The obtention of high-power (or high-energy) and high-luminance laser sources is currently limited by the flux stability of gain materials. One solution to this problem is to distribute amplification across multiple gain media in parallel. This requires the laser beams output from each gain medium to be in phase so as to ensure optimum coherent combining of all of the laser beams. It is therefore necessary to dynamically compensate for the delays introduced across a large number M of laser beams due to propagation through an assembly of gain media (fiber amplifiers for example) connected in parallel. Once phase-locked, the M nascent laser beams constructively interfere and thus form a source whose luminance is M times greater than that of an elementary amplifier, while retaining its beam-like quality (limited by diffraction in the case of single-mode fibers, for example). It is therefore a question of setting up as many phase-locked loops as there are emitters.
The architectures for phasing laser sources may be classified according to multiple criteria. The first is the manner in which the beams are spatially combined, or superposed. Two families are thus distinguished:                Tiled-aperture combining: the M laser beams are collimated and have parallel directions of propagation. This combining mode is the optical equivalent of a radar beamforming antenna. Tiled aperture has then an intense main lobe and parasitic side lobes.        Filled-aperture combining: the M beams are superposed in the near field by using polarizers or a diffractive optical element (DOE). The advantage of the filled-aperture combining method is its efficiency, since in this case there are no side lobes in the far field.        
The nature of the error signal follows, along with the processing that will make it possible to counteract it across the phases between laser sources, and to optimize their coherent addition. Essentially four methods for the coherent combing of laser beams are distinguished, classed according to the quantity of information contained in the negative feedback signal:
The method referred to as the “hill-climbing” method: the error signal is simply formed by drawing off a fraction of the combined energy, which is maximized by varying the phases of the M channels (beams) to be combined. This technique is based on a gradient-based optimization algorithm with M−1 dimensions. The complexity rests, in this instance, on the processing algorithm, the error signal, which is a scalar signal, is extremely simple and low cost. The drawback of this method is the bandwidth of the loop, which varies by 1/M. This method therefore lends itself more to a small number of combined beams, typically fewer than 10.
The method referred to as the OHD method, for “optical heterodyne detection”. In this method, the error signal, which is composed of the measurement of the phase of each emitter with respect to a reference beam, is a vector signal; one detector per channel is used. The M measurements are taken in parallel via heterodyne mixing and demodulation. The drawbacks of this method are:                using RF components, which increases the cost per channel;        having recourse to a reference beam;        the error signal, which is measured before combining and which does not guarantee optimum combining quality: it does not make it possible to compensate for fluctuations in phase between the phase measurement plane and the combining plane. It is therefore necessary for the system to be calibrated.        
The method referred to as the LOCSET, or synchronous multi-dither, method. As for the hill-climbing method, this method uses a fraction of the combined energy as an error signal, but in this case, the contributions from the various channels are identified by “frequency-marking” each channel via RF modulation at a frequency specific thereto. The error signal for each beam is then obtained through heterodyne mixing using a reference beam. This method is advantageous since it requires only one detector, and the availability of rapid-phase modulators allows a large number of channels to be envisaged. On the other hand, it requires a large number of RF components in the negative feedback loop (mixers, modulators, etc.), thereby considerably increasing the cost per channel of the system. A similar signal is obtained by temporally sequentially modulating each of the beams, at the same frequency in this instance, but with a negative impact on the bandwidth of the system.
The method of directly measuring the phases between emitters, for which the error signal is a map of the phases extracted from the interferogram of the beams to be combined, interfering either with one another or with a reference beam. This direct interferometric measurement method is collective: all of the phases are obtained through the recording of a single image by a matrix sensor, and it therefore lends itself perfectly to a large number of emitters. The cost of the imager used is to be divided by the number of channels and is therefore not decisive. The bandwidth of the system may, on the other hand, be limited by the sensor used, especially in the infrared. This is not a fundamental limit, however. Lastly, as for the OHD method, the phase is measured before combining; it does not make it possible to compensate for fluctuations in phase between the phase measurement plane and the combining plane and therefore does not guarantee optimum combining quality. It is therefore necessary for the system to be calibrated.
FIG. 4 summarizes the prior art of coherent combining techniques. The grayed cells indicate the negative points of each method.
There is therefore currently no existing architecture for the coherent combining of laser beams that simultaneously satisfies the conditions of a loop bandwidth >1 kHz, a number of beams that is potentially 100, 1000 or even higher, operation without calibration (error signal in the combining plane) and low cost.